1. Field of the Invention
This invention relates to a method of controlling the rotational velocity of an information recording disk, more particularly, it relates to a method of controlling an information recording disk having one or more tracks which are concentric or spiral and in which information is recorded and/or reproduced, for example, by a laser beam. This invention also relates to an information recording disk such as an optical or optical magnetic disk which is useful in performing the method.
2. Description of the Prior Art
Generally, when information or data are recorded into, reproduced or erased from an information recording disk in terms of bit patterns formed in tracks of the disk by a reading/recording apparatus, the rotational velocity of the disk is kept constant. When the rotational velocity of a disk is to be kept constant, however, the length of a pattern corresponding to one bit (hereinafter referred to as "bit length") which is formed on the outer periphery of the disk must be longer than the bit length which is formed on the inner periphery, resulting in a low recording density on the outer periphery despite a high recording density on the inner periphery to cause a problem of a reduced recording density of the disk as a whole.
This problem can be solved by controlling the linear velocity of a disk so as to be constant. However, this technique involves a drawback which will be described. When this technique is to be conducted, the absolute position of a device for reading data (hereinafter referred to as "pickup") in the radial direction of the disk must be accurately known. Usually, the information about this absolute position (hereinafter referred to as "position information") is previously recorded in an address part. When any process such as recording is conducted onto a track of a disk, the pickup of a reading/recording apparatus is accessed to the track to be processed, and then the address information must be read out from the address part formed in the track to confirm whether the pickup has been accurately addressed to the objective track or not. In order to read the address information accurately, the disk should be rotated at a velocity which corresponds to the position of the pickup. In other words, the address part having the address information to be read out should move relative to the pickup at a fixed linear velocity. For this purpose, the following conditions are required:
(1) When the address information is read out, the disk rotates at a velocity which corresponds to the position of the address information in the radial direction.
(2) The address information is read out to control the rotational velocity of the disk.
These conditions (1) and (2) conflict with each other, resulting in that, conventionally, it has been difficult to perform the recording, reproduction, or erasure of information at a constant linear velocity.